The illustrative embodiments described in the present application are useful in systems including those using thermal printheads and more particularly are useful in systems including those for providing thermal printhead temperature management by preprocessing images for use with direct contact thermal printheads.
Direct contact thermal print heads are typically designed to produce heat using thermal printhead heating elements in order to activate thermal media such as a thermal media label stock. Such thermal media may be gray scale media or in some cases, color media. When used with a grayscale thermal media stock, the elements are heated to higher levels to produce a darker gray output on the thermal media label stock. The thermal printhead typically includes a linear array of resistive heating elements that are brought to increased temperatures using increased drive current. The thermal media passes over the linear array and portions of the media are activated due to the heat present at each heater element.
The typical thermal print head includes a heat sink thermally connected to the heating elements so that heating elements will more quickly cool when the drive current is removed. Thermal printhead elements may be heated relatively quickly, but cool down more slowly using a heat sink. Accordingly, the printhead temperature curve includes hysterisis. The printheads often include a thermistor that is used to measure ambient temperature at the printhead and provide feedback to the printhead processor so that the heating elements may be properly driven to achieve the desired heat and intensity on the thermal media.
The temperature hysteresis problem is more troublesome at higher printing speeds and may affect the quality of printing gray-scale or color images. For example, when a dark or high intensity pixel is printed, the print head uses a high current to achieve the heat required at the heating element for that particular thermal media. If the subsequent pixel is relatively light or low intensity, the heating element may have retained significant heat from the prior pixel printing cycle. Accordingly, the printer must compensate for the pre-heated condition of the print head in a process that is referred to as Thermal History Management. In such a situation, the printhead might not use as much drive current because the print element is already somewhat heated. The printer must also manage the overall pre-heating of the printhead heat sink that affects all nearby printing elements in a process that is referred to as Thermal or Power Management. The printhead typically includes local processing systems to perform such compensation routines and thus requires a more expensive printer controller that is capable of performing the required calculations.
Thermal printheads are available from several companies including Kyocera Industrial Ceramics Corp. of Vancouver, Wash. Such printheads are available in a variety of sizes and geometric configurations sand may be purchased having resolutions of approximately 200 through 600 dots per inch (dpi). For example, the Kyocera KSB320BA printhead includes a chip thermistor. The printheads may vary in widths including approximately 40 mm through 927 mm and in custom configurations may have narrower widths including 27 mm. Similarly, thermal printers and printheads are available from several companies including the P91DW printer available from Mitsubishi Electric of Irvine, Calif. Thermal printheads may be constructed using thick film fabrication techniques.
Thermal printer subsystems may include a thermal printhead and a control processor or ASIC. The control processors may perform thermal history management locally on the fly as an image is printed. However, such systems require additional components and/or software to perform such hardware real-time thermal history management. A print control device and method of printing using the device is described in U.S. Pat. No. 6,709,083 B2, issued Mar. 23, 2004 to Fukushima. Fukishima describes a hardware temperature management circuit and feedback scheme for controlling heating element temperature.
Many thermal printheads are designed to operate as generic printers using standard printer software drivers to accommodate arbitrary images that are sent to the printer. The prior art does not provide a system and method for providing thermal printhead thermal history management and compensation in an external device.